When the power flow goes in opposite direction then the wattmeter shows negative reading.It means, let initially power flow from P1 to P2 direction, wattmeter shows positive reading and when direction of power flows change to P2 to P1 wattmeter shows negative reading.

For example if wattmeter shows negative reading when connected to motor or generator, then there are two obvious mistakes. One you must have connected wrongly and the other the sense of winding is wrong.The first one can be easily verified. Even after verification of the first mistake the problem continues then connect negative of winding to common of wattmeter and -ve 2 other end of wattmeter. If the problem is solved, then the sense of winding has to be changed.

If you have seen the wattmeter ratings, it will be (300V,10A,UPF) (Voltage,
Current, Power Factor)
That is wattmeter needs voltage and current to measure power. So a
wattmeter consists of two coils pressure coil, and current coil.
Pressure coil measure quantity proportional to voltage and current coil measures
quantity proportional to current.

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